A plasma is a gas which is sufficiently ionized to conduct current freely. Plasma arc cutting refers to a process which employs a high temperature arc or plasma jet to achieve melting and vaporization of a conductive material such as steel plate. The process is carried out by transferring a constricted electric arc from a negatively charged electrode through an ionized high velocity gas to a positive charged conductive workpiece. Plasma arc cutting equipment refers to equipment used to cut electrically conductive material.
Plasma cutting equipment generally comprises two main subassemblies, a power supply, which converts AC power to DC power, and a torch. The power supply, which typically represents 80% of the size, weight, and cost of the equipment, has the greatest need for improvement in size, weight, cost, and reliability
In a switching power supply, the AC input is rectified and filtered, chopped by a high frequency transistor switch/transformer combination, and then rectified and filtered again. The operation of the transistor switch can be controlled by a pulse width modulator (PWM) circuit including a ramp oscillator driving a differential voltage comparator, an error amplifier and a voltage reference. The error amplifier compares the reference with a sample of the supply's output voltage or current. As the load increases, the output drops; this drop is sensed in the error amplifier which causes the PWM, and consequenly the transistor switch, to remain ON for a longer time period. As a result, more energy is transferred to the transformer and the output of the supply increases. An overview of power supplies of this type appears in Section 6 "Switching Power Supply Design" of Vol. 1 of Linear and Interface Circuit Applications by D. E. Pippenger and E. J. Tobaben (Texas Instruments, 1985).
A switching supply does not need the relatively heavy and expensive power line transformer required by a linear power supply, the other type of power supply in common use. Besides the advantages of lighter weight, smaller size and lower cost, a switching supply is more electrically efficient since the transistor switch is either completely ON or OFF and the switch is operated at a frequency much higher than the AC line frequency. Switching supplies also generate more electromagnetic interference than linear supplies but this effect can be significantly reduced by, for example, filtering the input and output of the supply. Another problem that can occur due to the extremely high frequency components of a fast rising signal is "ringing", that is, the trapping and circulation of electrical energy in LC circuits created by the inherent inductance of conductors and the equivalent circuit gate to source capacitance of the power-switching transistors. If a transformer is connected to the switches, ringing can also be produced due to the parasite capacitance inherent in the transformer windings. Circulating electrical energy in such LC circuits is a problem since it introduces an uncertainty in the timing of the switching of the transistors.
In current designs, the power available from a switching supply is limited by the current and power capacity of the transistor switches used in the power output stage of the power supply. For example, field effect transistor (FET) H-bridges are commonly used in the power switching stages of switching power supplies. Although high power FET's are available, these transistors cannot provide unlimited power. Typically, on the order of 1 kilowatt can be provided by a single H-bridge using high power FETs. Where higher powers are required, H-bridge switches are typically connected in parallel. This arrangement has some disadvantages, however. In order to equalize the power flowing through the parallel H-bridge switches, the transistors used in the bridge are carefully matched, and additional circuitry may also be required to balance the power flowing through the individual switches. It would be advantageous to have a method of increasing the power available from a switching power supply by paralleling power output stages without the necessity (and cost) of using matched transistors or additional circuitry to ensure equal division of the output power between the parallel power stages. It would also be advantageous to have such a power supply which operates well with a plasma arc cutting torch as the load. A plasma arc torch is a difficult load because the load can vary greatly, the arc generates a high level of electromagnetic interference, and the plasma can be snuffed out by transient fluctuations in the power feeding the plasma.
Finally, it should be noted that pulse transformers with coaxial winding are known, but heretofore they have been used typically for VHF signal transmissions where the transformer changes the impedance between two very different levels. Also, more generally, transformers have been used as transistor gate drives. One example is a fast switch for a grid pulse modulator of a traveling wave tube amplifier. This application, which was used for electronic warfare countermeasures, involved a transformer with multiple secondaries that each drove one of a set of series-connected 2N 2222 bipolar transistors. To the best of applicants knowledge, however, a pulse transformer acting under the control of a pulse width modulator has never been used to control the synchronous switching of multiple H-bridges connected in parallel and each feeding a separate primary of a common power transformer as a way of paralleling the H bridges while still maintaining a synchronization of their switching.
It is therefore a principal object of the present invention to provide a regulated power supply that simultaneously couples a control signal to multiple switching circuits, not merely to multiple discrete switches, to provide higher output power levels than have been attainable heretofore from single bridge power supplies of this type.
Another principal object is to provide a power supply with the foregoing advantages that is comparatively inexpensive to manufacture and can use inexpensive, non matched MOSFET transistors in its switching circuits.
Yet another object is to provide a power supply with the foregoing advantages which operates reliably and with good efficiency.
A further object is to provide a power supply with the foregoing advantages that operates well with a plasma arc torch as a load, exhibits good maintenance characteristics, and has good adjustability.
Still another object is to provide a power supply with the foregoing advantages that is modular for convenient expansion or contraction of the maximum power rating of the supply, and does not require expensive, or performance restrictive, circuitry to balance the power flow through the modules.
A yet further object of the invention is to provide a switching power supply with the foregoing advantages that is compact, lightweight, and has good heat generation and dissipation characteristics.